The lac repressor and operator represent the most favorable system for studying the association of a protein with its specific DNA site. Recent studies have revealed that the structure of the DNA changes when lac repressor binds. Our preliminary results indicate that the structure of the repressor also changes at this time. Quite likely, these conformational changes maximize the interaction between the protein and DNA, providing for their very tight binding. We propose to study these changes, and the role the play in the association process. This information will help reveal structural features of the protein which enable it to recognize the unique base sequence of the lac operator. The lac repressor protein will be selectively modified with a variety of probes, mostly fluorescent, which are sensitive to local changes in structure. Some probes will be attached to the amino terminal region of the protein, which is thought to bind directly to the DNA. Other probes will be attached to protein "core". These probes will be used to report conformational changes when the repressor binds to poly(d(A-T)) DNA. These spectral changes will then be investigated by stop-flow fluorimetric techniques. The rates of individual reactions will be determined. These data will reval reactions which are occurring within a preformed repressor-poly(d(A-T)) complex.